The problem with the system's could be because of a virus or a spyware.
So scan the computer and if its clean of virus and spyware then we need to do a clean up job to improve the performance of the system.
1- Uninstall the unwanted programs and games that you don't use or don't wish to use in future, go to control panel -add/remove programs and you will find programs that you frequently use and the one's that you have rarely used, select the programs that you have rarely used and then then remove them.
Note: Uninstall only those programs that you know that you installed and are rarely used.
2- Delete the contents of the Prefetch, temp and the %temp% folder.
Click start-run- type the name of the above folders one by one and once that folder opens, select all the contents of the folders and press Shift+delete. You may not be able to delete all the files in the temp and the % temp% folder so don't worry about that, get rid of as many files as you can.
3-Run disk clean up, click start-all programs-accessories-disk clean up. Once the disk clean up window is open, go to the more option tab and click the clean up button under the System restore section.
Note: This will delete all the system restore point to free up a lot of hard drive space however, you won't be able to do system restore till the system creates a new restore point . So its upto you to decide.
3- Then come under the general tab of the disk clean up window and select all the boxes and press ok.
4- Increase the virtual memory . Go system properties, advanced tab, click settings under the performance tab and then to to the advanced tab and then click change under the virtual memory section. Then click the "Custom size" button and in the box that says initial size enter a value twice of your physical RAM, if you have 512 MB of physical RAM then enter 1024 in the initial size box and in the maximum size box enter four times the value of your physical RAM that is 2048, then click set and then click ok.
The system will ask for a restart, do not restart because there is one more step left.
5- Click start- run- msconfig. Go to the start up tab and uncheck the items you don't want to load when the system starts up, you will find a lot of such programs like yahoo messenger etc. There are some unwanted services that you can stop under the services section however, I would advice not to touch that section untill you are sure what all services to stop.
Restart the system and then check the performance of the system and you will notice a major change provided that there is no spyware of virus infection.
Good Luck!!